Ibn Hibban

Sayyida Aisha Siddiqa (may Allah be pleased with her) narrates: One day in the month of Ramadan, the Noble Prophet (peace and blessings be upon him) came out (i.e., to the mosque). He performed the (Tarawih) prayer, and some people, following his prayer, performed the prayer behind him. The next morning, people discussed this, so (the next day) the number of people increased. The Noble Prophet (peace and blessings be upon him) came out the second day as well, and people performed the prayer following his lead. The next day, people discussed this, so on the third night, many people gathered in the mosque. The Noble Prophet (peace and blessings be upon him) came out. People performed the prayer following his lead. When the fourth night came, the entire mosque was filled. That day, the Noble Prophet (peace and blessings be upon him) did not come out; he came to the people to perform the Fajr prayer. When the Fajr prayer was completed, he turned towards the people. He recited the testimony of faith and then said: "To proceed! Your gathering here was not hidden from me, but I feared that it would be made obligatory upon you and you would not be able to perform it." (Sayyida Aisha, may Allah be pleased with her, narrates:) The Noble Prophet (peace and blessings be upon him) used to encourage people to perform voluntary prayers in the month of Ramadan, however, he did not give them any formal command regarding this. He used to say to others: "Whoever, in a state of faith and hoping for reward, performs voluntary prayers in Ramadan, his previous sins will be forgiven." Then, when the Noble Prophet (peace and blessings be upon him) departed from this world, this matter remained as it was. Then, during the caliphate of Sayyiduna Abu Bakr (may Allah be pleased with him), this matter remained as it was, and in the early period of Sayyiduna Umar (may Allah be pleased with him) as well. (That is, after this, people began to perform the Tarawih prayer in congregation.)