Are there any scriptures that portray Lord Rama to be the source of all incarnations including Lord Vishnu?
Yes, there are scriptures that are used by Ramanandi Vaishnavas to support their claim of Lord Rama being the Original Supreme Personality of Godhead.
raghavasya guno divyo mahavisnuh svarupavana ।
vasudevo ghanibhutastanutejah sadasivah ।।
matsyasca ramahrdayam yogarupi janardanah ।
kurmascadharasaktisca varaho bhujayorbalam ।।
narasimho mahakopo vamanah katimekhala ।
bhargavo jamnghayorjato balaramasca prsthatah ।
bauddhastu karuna saksat kalkiscittasya harsatah ।।
krsnah srrngararupasca vrndavanavibhusanah ।।
aite camsakalah sarve ramo brahma santanah । (Sudarshana Samhita 1-5)
English Translation: When the infinite divine auspicious qualities of Lord Rama takes form, He becomes MahaVishnu. Shri Rama's auspicious condensed divine splendour is Lord Vasudeva. Matsya avatara appeared from Shri Rama's heart, Kurma avatara is the manifestation of His support-power. Varaha is the manifestation of His arm's strength. Lord Narasimha appears from His anger, Lord Vamana from His waist, Lord Parashurama appears from His thigh, Buddha from His compassion, Kalki appears from His delight and the Vrindavana-Vihari Shri Krishna Himself is His Shringaar Swaroopa. Thus, all these incarnations are are either plenary portions or portions of the plenary portions of Lord Rama and Lord Rama is the origin, the eternal Brahman and the original Supreme Personality of Godhead Himself.
yatha sarvavataranamavatari raghuttamah ।
tatha strotasam saumya pavini sarayu sarita ।। (Agastya Samhita, Uttara-Khanda)
English Meaning: Shri Rama, the best among the Raghus, is the original Supreme Personality of Godhead, among all the incarnations; in the same way as the divine river Sarayu is the greatest among all rivers.
avatarastu bahavah kala amsavibhutayah ।
ramodhanurdharah saksat sarveso bhagavan svayam ।। (Shiva Samhita)
English Translation: Various incarnations are either plenary portions or portions of the plenary portions of the Lord, but Lord Rama who wields the bow is the God of all Gods and the original Supreme Personality of Godhead Himself.
purnopurnavatarasca syamoramoraghuttamah ।
amsanrsimhakrsnadyah raghavobhagavansvayam ।। (Yajnavalkya Samhita)
English Translation: Lord Rama is the best among the Raghus is the original Supreme Personality of Godhead and also a complete incarnation. Narasimha, Krishna and others the plenary portions of Raghava, the original Supreme Personality of Godhead.
nārāyaṇō'pirāmāṃśaḥ-śaṅkhacakragadādharaḥ॥ (Varaha Samhita)
English Translation: Lord Narayana, the wielder of Shankha, Chakra, Gada and Padma is the first manifestation of Bhagavan Shri Rama.
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