Sayyiduna Ubadah bin al-Samit (may Allah be pleased with him) reports from the Prophet (peace and blessings be upon him): The Prophet (peace and blessings be upon him) said: Shall I not tell you about the things that become an expiation for sins? They said: Yes indeed, O Messenger of Allah! The Prophet (peace and blessings be upon him) said: Performing ablution thoroughly at times when it is hard on oneself, going to the mosques with many steps, and waiting for one prayer after another; this is ribat (preparedness, or guarding the frontier).
Imam al-Tabarani recorded it, and Imam al-Bazzar recorded something similar. Imam al-Bazzar's teacher, Khalid bin Yusuf al-Samti, transmitted it from his father, and they are da'if (weak); and the narrator Ishaq did not live in the time of Sayyiduna Ubadah (may Allah be pleased with him).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2125