2321 — Humayd ibn Mas'adah narrated to us, 'Abd al-Wahhab narrated to us, Ayyub narrated to me, who said:
'Umar ibn 'Abd al-'Aziz wrote to the people of Basrah: "It has reached us from the Messenger of Allah, peace and blessings be upon him, similar to the hadith of Ibn 'Umar, from the Prophet, peace and blessings be upon him," and he added: "And the best that can be estimated for it is that when we see the crescent of Sha'ban for such-and-such, then the fast, Allah willing, is for such-and-such, unless you see the crescent before that."(1)

2322 — Ahmad ibn Mani' narrated to us, from Ibn Abi Za'idah, from 'Isa ibn Dinar, from his father, from 'Amr ibn al-Harith ibn Abi Dirar,
from Ibn Mas'ud, who said: "When we fasted with the Prophet, peace and blessings be upon him, twenty-nine days was more frequent than what we fasted with him of thirty."(2)

--------------------------------------------

It was also recorded by al-Bukhari (1900), Muslim (1080), Ibn Majah (1655), and al-Nasa'i (2441) via the chain of Salim ibn 'Abdullah, and by al-Bukhari (1907) and Muslim (1080) via the chain of 'Abdullah ibn Dinar, both from Ibn 'Umar, with it.
It is also in "Musnad Ahmad" (4488), and "Sahih Ibn Hibban" (3441), (3445), (3451), and (3593).

His saying: "ghumma 'alaykum" (it was obscured from you) comes from your saying: ghamamtu al-shay' (I covered something) when you cover it; so it is maghmum (covered).

His saying: "faqduru lahu" (then estimate it) means: estimating it by completing the count to thirty. It is said: qadartu al-shay' aqdiruhu qadran — meaning qaddartuhu taqdiran (I estimated it). And from it is the saying of the Exalted: {Then We determined, and excellent are those who determine} [al-Mursalat: 23].

(1) Its narrators are trustworthy, but it is interrupted (munqati'). 'Abd al-Wahhab: he is Ibn 'Abd al-Majid al-Thaqafi.
See what precedes it.
Al-Mundhiri said in "al-Mukhtasar" 3/211: "And this which 'Umar ibn 'Abd al-'Aziz said is what the established narrations from the Messenger of Allah, peace and blessings be upon him, have ruled."

(2) Sahih by virtue of others, and this chain is weak due to the unknown status of Dinar, the father of 'Isa. Ibn Abi Za'idah: he is Yahya ibn Zakariyya al-Hamdani al-Wadi'i.