Yahya ibn Sam is classified by al-Hafiz as acceptable. Ahmad (5/150) narrated: Sufyan narrated to us that two men narrated to us from Musa ibn Talhah, Muhammad ibn Abd al-Rahman, and Hakim ibn Jubayr—from Ibn al-Hutaykiyyah, from Abu Dharr—that he said: He then cited it in similar wording.

In another version, he narrated: Sufyan said: “We heard it from two or three [narrators]: Hakim ibn Jubayr narrated to us from Musa ibn Talhah.” Al-Nasaʾi likewise narrated it, and then cited further variations in the reports from Musa ibn Talhah. Ibn Abi Hatim mentioned some of these in al-ʿIlal (1/267), but did not state which of them he regarded as preponderant. However, the hadith has a strong supporting report through Hammam, who said: Anas ibn Sirin narrated to us; Abd al-Malik ibn Qudamah ibn Milhan narrated to me from his father, who said: “The Messenger of Allah (peace and blessings be upon him) would instruct us to fast on the days of the bright white nights: the thirteenth, fourteenth, and fifteenth.”

Al-Nasaʾi and al-Bayhaqi narrated it through Anas ibn Sirin from him. Ahmad (5/27) likewise narrated it, but this Abd al-Malik is of unknown status; and his father is also said to have been Qatadah ibn Milhan. Ibn Hibban (946) also narrated it by way of Shuʿbah: Anas ibn Sirin narrated to me: I heard Abd al-Malik ibn al-Minhal ibn Milhan narrate from his father in similar wording. Ahmad (5/28) likewise narrated it, except that he did not say, “ibn Milhan.” Al-Bayhaqi also narrated it and said: “We have related from Yahya ibn Maʿin that he said: ‘This is an error. Rather, he is Abd al-Malik ibn Qatadah ibn Milhan al-Qaysi.’” That is, as in Ahmad’s earlier narration.

In sum, the hadith, taken together with all these chains, is good at the very least. And Allah knows best.