Abd al-Rahman — he is Ibn Awf(1) — on the authority of Abu Hurayrah, who said: The Messenger of Allah, may Allah bless him and grant him peace, said: "The most virtuous fasting after the month of Ramadan is the sacred month of Allah, al-Muharram, and the most virtuous prayer after the obligatory prayer is the night prayer."(2)

1614 — Suwayd ibn Nasr informed us, saying: Abdullah informed us, saying: Shu'bah narrated to us, on the authority of Abu Bishr Ja'far ibn Abi Wahshiyyah, that he heard Humayd ibn Abd al-Rahman(3) say: The Messenger of Allah, may Allah bless him and grant him peace, said: "The most virtuous prayer after the obligatory prayer is standing in prayer at night, and the most virtuous fasting after Ramadan is al-Muharram."(4)

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(1) His statement: "he is Ibn Awf," is not in (Q). Al-Hafiz said in "al-Nukat" (in the margin of al-Tuhfah 9/336): His statement: "Ibn Awf" is an error from someone other than al-Nasa'i. Ibn al-Sunni did not narrate it with the words "Ibn Awf" in it, and Muslim attributed him in one narration as "al-Humayri."
(2) Its chain of transmission is sound. Abu Awanah: he is al-Waddah ibn Abdullah al-Yashkuri. Shu'bah differed from him — as in the following narration — and narrated it from Abu Bishr — who is Ja'far ibn Iyas Abu Wahshiyyah — from Humayd ibn Abd al-Rahman, from the Prophet, may Allah bless him and grant him peace, as a mursal (incomplete chain), but al-Daraqutni authenticated its connected form in "al-Ilal" 9/91. It is in "al-Sunan al-Kubra" under numbers (1314) and (2919), and in neither of them is his statement: "he is Ibn Awf."
It was also narrated by Muslim (1163): (202), Abu Dawud (2429), and al-Tirmidhi (438) and (740), all three from Qutaybah, with this chain of transmission. In Muslim and al-Tirmidhi: Humayd ibn Abd al-Rahman al-Humayri.
It was also narrated by Ahmad (8534) and Abu Dawud (2429) through two paths from Abu Awanah, with it.
It was also narrated by Ahmad (8026), (8358), (8507), and (10915), Muslim (1163): (203), the author in "al-Kubra" (2917) and (2918), and Ibn Majah (1742) through the path of Muhammad ibn al-Muntashir, from Humayd, with it. See the disagreement over its chain of transmission in "al-Ilal" by al-Daraqutni 9/89-93.
Al-Sindi said: His statement: "the month of Allah" means: fasting the month of Allah. It was said: what is meant is fasting the day of Ashura, not fasting the entire month.
(3) After it in (R) and (M): "from Abu Hurayrah," which is an intrusive addition.
(4) A sound hadith; the discussion of it has preceded in the previous narration. Abdullah: he is Ibn al-Mubarak. It is in "al-Sunan al-Kubra" under number (1315).