Where did the Buddha speak of his difficulty in eating almsfood as a Bodhisatta?
I found it in the introduction to the Jatakas, which seems to be the original source:

Now the Great Being, after collecting a number of scraps, sufficient, as he judged, for his sustenance, left the city by the same gate he had entered, and sitting down with his face to the east, in the shade of Pandava rock, he attempted to eat his meal. But his stomach turned, and he felt as if his inwards were on the point of coming out by his mouth. Thereupon, in the midst of his distress at that repulsive food, --for in that existence he had never before so much as seen such fare,--he began to admonish himself, saying, &quot;Siddhattha, although you were born into a family having plenty to eat and drink, into a station in life where you lived on fragrant third season's rice1 with various sauces of the finest flavors, yet when you saw a monk clad in garments taken from the rubbish heap, you exclaimed, 'Oh, when shall I be like him, and eat food which I have begged? Will that time ever come?' And then you retired from the world. And now that you have your wish, and have renounced all, what, pray, is this you are doing?&quot; When he had thus admonished himself, his disgust subsided, and he ate his meal.
-- Jāt-A nidānakathā (from Buddhism In Translations, Henry Clarke Warren, trans.)

The relevant Pali is &quot;antāni parivattitvā mukhena nikkhamanākārappattāni viya ahesuṃ&quot; - It was like the intestines, having churned, attained the state of leaving through the mouth.&quot;
The Buddhavamsa doesn't mention this reflection, saying of his first alms round simply:

nisīditvā āhāraṃ paccavekkhitvā nibbikāro paribhuñji
Having sat down, having reflected on the food, unperturbed, he ate.
-- Bv-A, nidānakathā