One of the teachers under whom Gotama, after leaving the world and before he became the Buddha, received instruction. Uddaka taught him the doctrine which had been realised and proclaimed by Rāma, which was the attainment of the state of “neither-perception-nor-non-perception”. When Gotama had mastered this, Uddaka made him more than his own equal by setting him over the whole company of his disciples as their teacher. But Gotama, finding this doctrine unsatisfactory, abandoned it. MN.i.165ff. MN.i.240ff.
The Buddha evidently had a high regard for Uddaka-Rāmaputta, for after the Enlightenment, when looking for someone to whom the Dhamma might be preached, and who was capable of realising its import at once, his thoughts turned to Uddaka, but Uddaka was already dead. Vin.i.7
In the Vassakāra Sutta of the Aṅguttara Nikāya AN.ii.180 it is mentioned that King Eḷeyya, together with his bodyguard, Yamaka, Moggalla and others, were followers of Rāmaputta and that they held him in great esteem.
In the Saṃyutta Nikāya SN.iv.83f. the Buddha says that Uddaka claimed to be “versed in lore and to have conquered everything, digging out the root of Ill,” though he had no justification for such a claim.
Again, in the Pāsādika Sutta DN.iii.126–127 the Buddha tells Cunda that when Uddaka said “seeing, he sees not,” he had in mind a man who saw the blade of a sharpened razor but not its edge—a low, worldly thing to speak about.