Musannaf Ibn Abi Shayba 1:1332

Humaid ibn Abd al-Rahman narrated to us, from Sinan al-Barjami, from Mughira, from Ibrahim, regarding a man who spits and there is blood in his saliva: He said: "If the redness overcomes the whiteness, perform ablution (wudu). And if the whiteness overcomes the redness, do not perform ablution."