I see this Shloka in many places which means "as you think so you become". The Sanskrit verse is Yad bhavam Tad bhavathi.
Is there a scriptural source for this verse?
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Upon searching on web or through Sanskrit Documents, I've not found the exact verse "यद् भावं तद् भवति". However, worth to mention that following couple of verses sounds to be very closer I found from the discussion at IndianDivine.org.
Rigveda 10.90.2 (Purusha Sukta):
पुरुष एवेदं सर्वं यद्भूतं यच्च भव्यम्।
उतामृतत्वस्येशानो यदन्नेनातिरोहति॥ १०.०९०.०२
purusha evedaGM sarvam | yad bhUtam yac ca bhavyam |
utAmRtatvasyeshAnaH | yad annenAtirohati || 2 ||
The term "यद् भावं तद् भवति" may be the modification of "यद् भुतं यच्च भव्यम्"
Brihadaranyaka Upanishad 1.5.18:
पृथिव्यै चैनमग्नेश्च दैवी वागाविशति । सा वै दैवी वाग्यया यद्यदेव वदति तत्तद्भवति ॥ १८ ॥
I-v-18: The divine organ of speech from the earth and fire permeates him. That is the divine organ of speech through which whatever he says is fulfilled.
The 8.6th verse of Bhagavad Gita conveys similar message to "as you think so you become":
8.6 O son of Kunti, thinking of any entity whichever it may be one gives up the body at the end, he attains that very one, having been always engrossed in its thought.
Which is also said by Shandilya (Chhandogya Upanishad 3.14.4)
Hence the exact term "यद् भावं तद् भवति" may either be emerged from this Vedic or Upanishadic preaching or be a part of Shubhashita or be found in Bhashya or works of Aacharya which requires further investigation.