It is narrated from Sayyiduna Rifa'ah al-Ansari (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "In Fajr, no fewer than twenty verses should be recited, and in Isha, no fewer than ten verses should be recited."
Hadith Referenceالمعجم الكبير للطبراني / باب الراء / 4523