Valmiki Ramayana Sundara Kanda Sarga 1 Shloka 188
Original Shlokaस तां बुद्ध्वार्थतत्त्वेन सिंहिकां मतिमान्कपिः।।5.1.188।। व्यवर्धत महाकायः प्रावृषीव वलाहकः।
1 source checked for this shloka
मतिमान् wise one, सः कपिः Hanuman, ताम् her, अर्थतत्त्वेन by her true nature, सिंहिकाम् Simhika, बुद्ध्वा recognising, महाकायः huge bodied one, प्रावृषि in rainy season, वलाहकः इव like a cloud, व्यवर्धत grew.
Recognising the true nature of Simhika the clutcher of shadows, the wise Hanuman grew to a huge size like a cloud in rainy season.