Ramadan

Abdur Rahman bin Abd Qari narrates that one night, during the month of Ramadan, he went with Hazrat Umar (ra) to the mosque and saw that people were scattered in various groups whilst offering their prayers. Some were offering the prayer on their own, whilst others were offering prayers with a small congregation behind them. Upon seeing this, Hazrat Umar (ra) stated that he deemed it better if everyone was brought together as one congregation behind a single qari [Imam]. Upon making this decision, Hazrat Umar (ra) appointed Hazrat Ubayy (ra) bin Kaab to lead the congregation.

(Sahih al-Bukhari, Kitab al-Salat al-Tarawih, Bab Fadl man Qam Ramadan, Hadith 2010, Translated version, Vol. 3, 680-681, Nazarat-e-Ishaat Rabwah)

In other words, the Muslims were offering their nawafil [voluntary] prayers at night.

Hazrat Anas (ra) relates that during the lifetime of the Holy Prophet (sa), Hazrat Abu Talha (ra) would refrain from voluntary fasting as he wanted to preserve his strength when engaging in Jihad. Hazrat Anas (ra) further relates that after the Holy Prophet (sa) passed away, he observed that Hazrat Abu Talha (ra) kept fasts every single day except on Eid-ul-Fitr and Eid-ul-Adha. In other words, he started regularly observing fast after this.

(Sahih al-Bukhari, Kitabul Jihad wa as-Siyar, Bab Man Ihktara al-Ghazwa alaa as-Saum, Hadith 2828)