مِشْكَاةُ الْمَصَابِيحِ
Mishkat al-Masabih 1291
Prayer
Humble Supplication - Section 2
وَعَنْ أَنَسٍ: أَنَّ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَنَتَ شَهْرًا ثُمَّ تَرَكَهُ. رَوَاهُ أَبُو دَاوُد وَالنَّسَائِيّ
Anas said that the Prophet (ﷺ) stood in humble supplication for a month and then gave it up. Abu Dawud and Nasa'i transmitted it.
Mishkat al-Masabih, 1291صَحِيح (الألباني)