Ikrimah (may Allah have mercy on him) said: A man asked Sayyiduna Ibn Abbas (may Allah be pleased with them both): May I eat the spleen? He said: Yes. The man said: Most of it is blood. Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: Only flowing blood is unlawful.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19699