Sayyiduna Abu al-Darda' (may Allah be pleased with him) narrates: I heard the Prophet (peace and blessings be upon him) say: "Whoever, every day, 27 times (the narrator is unsure, or perhaps the words are:) 25 times (it is one of these two numbers; whoever this many times every day) seeks forgiveness for the believing men and the believing women, he becomes one of those whose supplications are answered, and because of whom the people of the earth are given provision."
At-Tabarani recorded it; its chain contains a narrator, Uthman bin Abi al-Atikah, and in this narration he said: 'It was narrated to me from Sayyidah Umm al-Darda' (may Allah be pleased with her).' This narrator Uthman has been declared thiqah (trustworthy) by more than one, while the majority declared him da'if (weak); the other narrators named in its chain are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب التوبة / 17600