Hadith #2436 (Sunan Abi Dawud)
Book: Fasting (Kitab Al-Siyam)
Narrator(s): al-Hur bin al-Sabah الحر بن الصباح النخعي, Hunayda bin Khalid, Masdad bin Masrhad مسدد بن مسرهد بن مسربل, Wadah bin 'Abdullah al-Yashkari وضاح بن عبد الله اليشكري أبو عوانة
Grade: Sahih

The client of Usamah ibn Zayd said that he went along with Usamah to Wadi al-Qura in pursuit of his camels. He would fast on Monday and Thursday. His client said to him: Why do you fast on Monday and Thursday, while you are an old man? He said: The Prophet of Allah (ﷺ) used to fast on Monday and Thursday. When he was asked about it, he said: The works of the servants (of Allah) are presented (to Allah) on Monday and Thursday.

Source: https://parallelquran.com/hadith/sunan-abi-dawud-2436/