According to Wiki: "Bhakti in Indian religions is "emotional devotionalism", particularly to a personal god or to spiritual ideas"
Do any principal upanishads recommend devotion to a personal God?
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There are 2 views on this issue. Western Indologists and Indian scholars trained in western ways say no.
Vedantic teachers say yes.
Mental activities relative to the Saguna Brahman - such as are described in the Shandilya Vidya are Upasanas or devotions.
Vedantasara 12 of Sadananda Yogindra
So where is Shandilya Veda? It is in Chandogya Upanishad.
Verily, all this universe is Brahman. From Him do all things originate, into Him do they dissolve and by Him are they sustained. On Him should one meditate in tranquility. For as is one's faith in this world, such one becomes on departing hence. Let one, therefore, cultivate faith.
Chandogya Upanishad 3.14.1
The bolded mantra in the above Chandogya shloka is considered by Sri Yogananda as Upanishadic support for Bhakti marga.