The third foundational principle of worship is consistency.

Abu Huraira reported: The Messenger of Allah ﷺ said, "Take up good deeds only as much as you are able, for the best deeds are those done regularly even if they are few."
(Sunan Ibn Mājah 4240, authenticated by Al Albani)

Narrated 'Alqama: I asked 'Aisha, mother of the believers, "O mother of the believers! How were the deeds of the Prophet? Did he use to do extra deeds of worship on special days?" She said, "No, but his deeds were regular and constant, and who among you is able to do what the Prophet ﷺ was able to do (i.e. in worshipping Allah)?"
(Sahih al-Bukhari 6466)

Allah loves the slave to be consistent upon what he does even if it is little because that which a person is consistent upon is far greater than that which a person does and then leave off even if it is a lot. Thus, the poet said

اطلب ولا تضجر من مطلب ... فآفة الطالب أن يضجرا

Go and seek knowledge and don't give up,
the most harmful for a student of knowledge is to give up and stop.

أما ترى الماء بتكراره ... في الصخرة الصماء قد أثرا

Do you not see with your two eyes,
water dropping on a rock until it finally pierces through it.

How did it do it? Consistency.

It kept dropping on that particular spot until it finally affected the rock and pierced it.

Our pious predecessors were the most consistent of people with regard to their deeds. Their level of conformity was to a whole new degree.

Umm Habibah, the wife of the Prophet, reported: The Messenger of Allah ﷺ said, "Whoever performs twelve Raka'ahs of prayer in each day and night, then by them a house will be built for him in Paradise."

Umm Habibah said, "I have never abandoned them since I heard it from the Messenger of Allah ﷺ."

'Anbasa (one of the narrators) said, "I have never abandoned them since I heard it from Umm Habiba."

'Amr bin Aus (another narrator) said, "I have never abandoned them since I heard it from 'Anbasa."

Nu'man ibn Saalim (another narrator) said, "I have never abandoned them since I heard it from 'Amr bin Aus."
(Sahih Muslim 728)

The Prophet ﷺ and our pious predecessors were people of consistency unlike us. These days, Muslims, unfortunately, lack this quality. We start something, only to do it for some time before giving up. Time goes by and we change, another thing comes to our attraction, so, we carry on doing that for a while, then leave it and jump to another and then leave it again. So, a person goes through ten, fifteen, twenty different phases yet he is not consistent upon anything.

Allah, the Exalted, when He describes a believer says,

الَّذِينَ هُمْ عَلَىٰ صَلَاتِهِمْ دَائِمُونَ

Those who are constant in their prayer
-Surah Ma'arij, Ayah 23

They are the ones who are consistent upon their prayers. They are not those who pray sometimes and leave. They are not Friday Muslims. They are people who are steadfast and regular in their deeds which is why Allah praises them here.

In another place, Allah says that Isa AS said,

وَجَعَلَنِي مُبَارَكًا أَيْنَ مَا كُنتُ وَأَوْصَانِي بِالصَّلَاةِ وَالزَّكَاةِ مَا دُمْتُ حَيًّا

And He has made me blessed wherever I am and has enjoined upon me prayer and Zakah as long as I remain alive
-Surah Maryam, Ayah 31

Allah also says,

وَاعْبُدْ رَبَّكَ حَتَّىٰ يَأْتِيَكَ الْيَقِينُ

And worship your Lord until there comes to you the certainty (death).
-Surah Hijr, Ayah 99

This shows us that a believer does worship on the mindset that he is going to carry it out until his last breath. There is no set time period where he starts and then leaves. Rather, he continues to stay consistent upon it until death because,

Narrated Sa'd bin Sahl As-Sa'idi: The Prophet ﷺ said, "Verily, the (results of) deeds done, depend upon the last actions."
(Sahih al Bukhari 6493)

We never know which action of ours could be the last. Hence, it is imperative that we remain consistent in our actions regardless of how small it is.