Sayyidina Abu Hurayrah reported that Allah’s Messenger encouraged people to offer (voluntary) salah during Ramadan without prescribing it as an obligation. He would say, “He who stands in prayer during Ramadan with faith and seeking reward sincerely is forgiven what has past of his sins”. Then Allah’s Messenger (SAW) died while this was practiced. Then it was done like that in the caliphate of Abu Bakr and the early period of the caliphate of Umar ibn Khattab (RA) in the same manner.
[Ahmed7792, Muslim 759, Abu Dawud 1371, Nisai 2100]