Sahih Muslim Book 9, Hadith Number 3576.
Chapter: Li’an (Invoking curse).
Abu Huraira (Allah be pleased with him) reported: A desert Arab came to Allah’s Messenger (may peace be upon him) and said: My wife has given birth to a dark-complexioned child and I have disowned him. Thereupon Allah’s Apostle (may peace be upon him) said: Have you any camels? He said: Yes. He said: What is their colour? He said? They are red. He said: Is there anyone dusky among them? He said: Yes. Allah’s Messenger (may peace be upon him) said: How has it come about? He said: Messenger of Allah, it is perhaps due to the strain to which it has reverted, whereupon the Holy Prophet (may peace be upon him) said: It (the birth) of the black child may be due to the strain to which he (the child) might have reverted.