Muwatta Imam Malik (Riwayat Ibn al-Qasim)

And it is narrated from (the same chain of narration from Sayyida Aisha, may Allah be pleased with her) that the Messenger of Allah, peace and blessings be upon him, prayed (the night) prayer (Tarawih) in the mosque, so (some) people prayed with him (behind him). Then on the following night, when he prayed, the people increased. Then on the third or fourth night, (many) people gathered, but the Messenger of Allah, peace and blessings be upon him, did not come out to them (to lead the prayer). When morning came, he, peace and blessings be upon him, said: "I saw what you did (last night), but I only did not come out to you because I feared that this (standing in prayer) might become obligatory upon you." This incident happened in Ramadan.