مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3316
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated from Ibn 'Abbas (رضي الله عنه) that The Prophet (ﷺ) made (a couple) engage in li’an when there was a pregnancy.
Musnad Ahmad 3339A saheeh hadeeth; this is a da'eef isnad]