Al-Qasim bin Muhammad narrates from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that a man came to the Messenger of Allah (peace and blessings be upon him) and said: "O Messenger of Allah! By Allah! I have had no relations with my wife since the palms were first watered." (The narrator) says: 'Afar is that when the palms are pollinated, they are left without water for forty days; they are not watered after pollination. The man said: "I found a man with my wife." Her husband was pale yellow, with thin shins and straight hair, while the man with whom she was accused was of deep black complexion with curly hair. The Messenger of Allah (peace and blessings be upon him) supplicated: "Allahumma bayyin (O Allah, make the matter clear)." Then li'an was carried out between the two of them, and the woman gave birth to a child resembling the man with whom she had been accused. Abu al-Zinad narrates from al-Qasim bin Muhammad, and he from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both), that he heard the Messenger of Allah (peace and blessings be upon him): he (peace and blessings be upon him) carried out li'an between al-Ajlani and his wife while the woman was pregnant, and the man with whom she had been accused was Ibn al-Sahma'.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب اللعان / 15349