The Messenger of Allah ﷺ used to fast during Shaban and Ramadan, and he would seek out Mondays and Thursdays for fasting.
أخبرنا عمرو بن علي قال حدثنا عبد الله بن داود قال حدثنا ثور عن خالد بن معدان عن ربيعة الجرشي
كان رسول الله ﷺ يصوم شعبان ورمضان ويتحرى الاثنين والخميس
[Its chain of transmission is sahih] Reported by Tirmidhi in Sawm, chapter on what has been said about fasting on Monday and Thursday, hadith no. 745, from the hadith of Abdullah bin Dawud, with this wording, and he said, hasan gharib. It is also in al-Kubra, hadith no. 2497.* Thawr is Ibn Yazid.