Halliday & Matthiessen (2004: 456, 457):
Here a thought is represented as if it was a wording, for example I thought 'I'll just inquire' … The implication is ‘I said to myself…’; and this expression is often used, recognising the fact that one can think in words. Only certain mental process verbs are regularly used to quote in this way, such as think, wonder, reflect, surmise.