The twenty distinguished acts of worship are as follows-

1. Ikhlaas:

One of the prime actions that our pious predecessors focused on was Ikhlaas; a trait, the importance of which can never be overstated. It is the foundation on which all good deeds are built.

Al-Fudayl b. ʿAyyāḍ [187H] – Allah have mercy on him – said:
"Allah accepts only those deeds which are both correct and sincere (pure). If the deed is done correctly but not sincerely, it will not be accepted. And if it is sincere but not correct, it will not be accepted." He was asked, "Abū 'Alī! What is the sincere and correct deed?" He replied, "The sincere deed is one that is done only for Allah. And the correct deed is one done according to the Sunnah."
(Abū Nu'aym, Hilyah Al-Awliyā' Vol.8 p95.)

The Salaf were so keen to preserve their sincerity that they made their good deeds a secret between them and Allah, seeking only His face. They used to hide it in a way we hide our sins.

Muhammad b. Wāsi', a famous and pious Tābi'īn, said:
I have lived amongst men who were such that one of them would lie with his wife on the same pillow and his side of the pillow would be soaked with his tears under his cheek without his wife even noticing.
(Ibn Abī Al-Dunyā, Al-Ikhlās wa Al-Nīyah (Sincerity and Intentions) p34.)

It is reported that when Ibn Abī Laylā prayed [at night], if someone entered [his house], he would lie down on his bed [as if he was sleeping].
(Abū Nu'aym, Ḥilyatu Al-Awliyā Vol. 4 p351.)

It is reported that Ayyūb Al-Sakhtiyānī used to pray all night and hide it. In the morning, he would raise his voice as if he had just woken up.
(Al-Dhahabī, Siyar A'lām Al-Nubalā. Vol. 6 p17.)

It is reported that Dāwūd b. Abī Hind fasted for forty years without his family knowing, he would take his lunch out with him and donate it in the street.
(Al-Dhahabī, Siyar A'lām Al-Nubalā, Vol. 6 p378.)

It is reported that Shurayḥ the Judge used to have a house in which he would spend time alone on Fridays; no one knew what he did in it.
(Al-Dhahabī, Siyar A'lām Al-Nubalā' Vol.4 p105.)

It is reported that Abū Al-'Āliyah said, "I learned writing and the Qur'ān without my family noticing, and not a drop of ink was ever seen on my garment."
(Abū Nu'aym, Ḥilyatu Al-Awliyā' Vol. 2 p217.)

Even something as subtle as emotions was carefully disguised and reserved exclusively to Allah. Their sincerity dripped in each and every action.

It is reported from Ḥammād b. Zayd that ʾAyyūb [Al-Sakhtiyānī] was once in a gathering when he was effected (moved to tears) by an exhortation, so he began wiping his nose saying, "How severe a cold can be!"
(Al-Dhahabī, Siyar ʾAʿlām Al-Nubalāʾ 5:20)

It is also reported from Ḥammād that ʾAyyūb was once overcome with weeping, so he said, "A shaykh, when he gets very old, dribbles."
(Al-Dhahabī, Siyar ʾAʿlām Al-Nubalāʾ, 5:22)

In fact, their sincerity continued to beautify them, until it became manifest in their actions.

It is reported that the son of Al-Fuḍayl b. 'Ayyāḍ – Allāh have mercy on him – said to his father:
Father! How sweet (beautiful) the speech of the Companions is! [Al-Fuḍayl] said, "Son, do you know why it was so sweet?" He replied, "No father, I do not." He said, "Because they sought Allāh the Exalted when they spoke."
(Al-Bayhaqī, Shu'ab Al-īmān 2:299)

And it was this pure sincerity that made their words and actions serve as a shining legacy for millions to inspire even today.