How about one who fasts one day and breaks his fast the next? He said: "That is the fast of Dawud, peace be upon him." He said: How about one who fasts one day and breaks his fast for two days? He said: "I wish that I were able to do that." Then he said: "Three days of every month, and Ramadan to Ramadan—this is fasting for the whole of one's life."(1)

‌‌76 - Chapter: Fasting one day and breaking the fast one day, and the mention of the differing wordings of the transmitters regarding the report of 'Abdullah ibn 'Amr on that

2388 - He said: Among what Ahmad ibn Mani' read to us, he said: Hushaym narrated to us, he said: Husayn and Mughirah informed us, from Mujahid, from 'Abdullah ibn 'Amr, who said: The Messenger of Allah, peace and blessings be upon him, said: "The best fasting is the fasting of Dawud, peace be upon him—he used to fast one day and break his fast one day."(2)(3)

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(1) Its chain is authentic; Hammad is Ibn Zayd. It is in al-Sunan al-Kubra, no. (2708).
Muslim (1162): (196) and al-Tirmidhi (767) both narrated it from Qutaybah, with this chain.
Muslim's version is expanded, and al-Tirmidhi's version is abridged.
It was also narrated—in expanded form—by Muslim (1162), Abu Dawud (2425), al-Tirmidhi (767), Ibn Majah (1713), and Ibn Hibban (3639) via multiple routes from Hammad ibn Zayd, from him.
It has preceded—in abridged form—in the two narrations (2382) and (2383).
Al-Sindi said: His words "Or can anyone bear that?"—it seems he disliked it because it is something most people are incapable of, so one should not be encouraged toward it in a religion that is easy and lenient.
"That is the fast of Dawud"—that is, and the fast of Dawud is the best of fasting, and it seems he left it aside due to his affirming it repeatedly.
"I am able to do that"—that is, able to do it while fulfilling the rights of women. So the reference here is to the fear of failing to fulfill the rights of women, for continuous fasting would impair their share of him; otherwise, he was able to do more than that, for he used to practice continuous fasting.
(2) In (M): "he used to break his fast one day and fast one day."
(3) Its chain is authentic; Hushaym is Ibn Bashir, and Husayn is Ibn 'Abd al-Rahman al-Sulami. =