Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) says: I do not regret anything that I missed as much as I regret fasting and prayer, and that I left the rebel group (I should have fought them), and (I regret) that I withdrew my pledge of allegiance to Sayyiduna Ali (may Allah be pleased with him).
Al-Tabarani recorded it in al-Mu'jam al-Kabir and al-Mu'jam al-Awsat, but with these words: "I do not regret anything of this world that I missed as much as I regret the fasts of the hot season, and that I did not spread my feet apart in prayer," meaning that he did not offer long prayers. Its chain contains a narrator, Sinan bin Harun, whom Abu Hatim and Ibn Adi declared thiqah (trustworthy), while Yahya bin Ma'in called him da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصيام / 5089