Musnad Abu Ya'la al-Mawsili 4:11

Ibrahim ibn al-Hajjaj al-Nayli narrated to us, Abu 'Awanah narrated to us, from Simak ibn Harb, from Musa ibn Talhah, from his father, who said: I passed by with the Messenger of Allah (may Allah bless him and grant him peace) by some people at the tops of palm trees. He said, "What are these people doing?" They said, "They are pollinating them; they put the male into the female, and it becomes pollinated." He said, "I do not think that will avail anything." So they took that [advice] and abandoned it. Then the Prophet (may Allah bless him and grant him peace) was informed of that, so he said, "If it benefits them, then let them do it, for I only conjectured a conjecture, so do not hold me accountable for conjecture. But if I inform you of something from Allah, then take it, for I will never lie about Allah regarding anything."