diff --git a/libs/langchain/langchain/chains/qa_with_sources/base.py b/libs/langchain/langchain/chains/qa_with_sources/base.py
index db07d24b233..d0408059a8e 100644
--- a/libs/langchain/langchain/chains/qa_with_sources/base.py
+++ b/libs/langchain/langchain/chains/qa_with_sources/base.py
@@ -120,9 +120,11 @@ class BaseQAWithSourcesChain(Chain, ABC):
 
     def _split_sources(self, answer: str) -> Tuple[str, str]:
         """Split sources from answer."""
-        if re.search(r"SOURCES:\s", answer):
-            answer, sources = re.split(r"SOURCES:\s|QUESTION:\s", answer)[:2]
-            sources = re.split(r"\n", sources)[0]
+        if re.search(r"SOURCES?[:\s]", answer, re.IGNORECASE):
+            answer, sources = re.split(
+                r"SOURCES?[:\s]|QUESTION:\s", answer, flags=re.IGNORECASE
+            )[:2]
+            sources = re.split(r"\n", sources)[0].strip()
         else:
             sources = ""
         return answer, sources
diff --git a/libs/langchain/tests/unit_tests/chains/test_qa_with_sources.py b/libs/langchain/tests/unit_tests/chains/test_qa_with_sources.py
index e69d9b5cd11..56daf006859 100644
--- a/libs/langchain/tests/unit_tests/chains/test_qa_with_sources.py
+++ b/libs/langchain/tests/unit_tests/chains/test_qa_with_sources.py
@@ -12,6 +12,21 @@ from tests.unit_tests.llms.fake_llm import FakeLLM
             "This Agreement is governed by English law.\n",
             "28-pl",
         ),
+        (
+            "This Agreement is governed by English law.\nSources: 28-pl",
+            "This Agreement is governed by English law.\n",
+            "28-pl",
+        ),
+        (
+            "This Agreement is governed by English law.\nsource: 28-pl",
+            "This Agreement is governed by English law.\n",
+            "28-pl",
+        ),
+        (
+            "This Agreement is governed by English law.\nSource: 28-pl",
+            "This Agreement is governed by English law.\n",
+            "28-pl",
+        ),
         (
             "This Agreement is governed by English law.\n"
             "SOURCES: 28-pl\n\n"