Riyad as-Salihin

Abu Hurairah (May Allah be pleased with him) reported: The Messenger of Allah (ﷺ) said, "The best month for observing Saum (fasting) next after Ramadan is the month of Allah, the Muharram; and the best Salat (prayer) next after the prescribed Salat is Salat at night (Tahajjud prayers)."

[Muslim].