A man came to Ibn Abbas asking him about fasting. Ibn Abbas said, "The Messenger of Allah ﷺ used to say, 'Among the best forms of fasting is the fasting of my brother Dawud. He would fast one day and break his fast the next.'"
حدثنا أبو النضر قال حدثنا الفرج بن فضالة عن أبي هرم
جاء رجل إلى ابن عباس يسأله عن الصيام فقال كان رسول الله ﷺ يقول إن من أفضل الصيام صيام أخي داود كان يصوم يوما ويفطر يوما
Its chain of transmission is daif, due to the weakness of Faraj bin Fadala, as we mentioned at 581. Abu Harm is unknown both as a person and in status. Al-Hafiz said in al-Taajil, 186-187, Ahmad related the hadith from the transmission of Faraj bin Fadala from Abu Harmuz, this is how it appears in the original text, with a dammah on the ha, a sakin on the ra, followed by a mim, then a dotted za. Husayni wrote it in his own hand, and those who followed him did so, without the za, which is what appears in the History of Ibn Asakir in the handwriting of the author's son. Ibn Asakir stated definitively that he is Abu Hurairah, the one from Homs, then he pointed to another, longer version of the hadith, in which it appears as being from Abu Hurairah al-Himsi. See also al-Taajil, 524-525. However, what appears in both original texts here is "from Abu Harm," and whichever it may be, he is unknown.As for Sadaqah al-Dimashqi, he is likewise not well known. Al-Hafiz, following Ibn Asakir, favored the view that he is Sadaqah bin Abdullah al-Samin. If that is indeed who he is, then he is daif and came too late to have met Ibn Abbas, and if not, then he is unknown.The hadith appears in Majma al-Zawaid, 3:193, where it says, Ahmad related it, and Sadaqah is daif, though he has been given some degree of authentication, and he did not meet Ibn Abbas. So he stated definitively that he is al-Samin, forgetting all the rest of the unknown and weak elements in the chain of transmission.