Hadith: The most excellent fast after Ramadan is Allah's month, Muharram, and the most excellent prayer after what is prescribed is prayer during the night

عن أبي هريرة رضي الله عنه قال: قال رسول الله صلى الله عليه وسلم : «أفضل الصِّيام، بعد رمضان، شَهر الله المُحَّرم، وأفضل الصلاة، بعد الفَريضة، صلاة الليل».
[صحيح] - [رواه مسلم]

Abu Hurayrah (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) said: "The most excellent fast after Ramadan is Allah's month, Muharram, and the most excellent prayer after what is prescribed is prayer during the night."
Sahih/Authentic. - [Muslim]

Explanation

Fasting in the month of Muharram, which is the first month in the Hijri calendar, is the best fast after that of Ramadan. This is because Muharram marks the beginning of a new year, so commencing it with fasting, which is a light, is the most virtuous deed. Therefore, a Muslim should fast during it and try not to miss it except for an excuse. The Prophet's statement "the month of Allah" highlights the significance and distinction of Muharram among the other months. The Hadīth also shows that praying at night is the best voluntary prayer a Muslim can offer after the obligatory ones. This is because in night prayers man achieves a high level of humbleness and submission as the heart is serene and in complete privacy with its Lord at these times. Allah, the Almighty, said: {Indeed, the hours of the night are more effective for concurrence [of heart and tongue] and more suitable for words} [Sūrat al-Muzzammil: 6]. On the other hand, the night is the time of tranquility and rest, and thus it becomes harder on the soul and tiring for the body when the person dedicates it to worship. Therefore, praying at night occupies the top of religious assignment and is the most excellent in the sight of Allah.